The conventional pencils are manufactured by depositing a graphite or pigmented lead inside a wood casing. The limited selection of suitable woods, the treatments that they have to undergo, the complexity of the steps of the manufacturing and finishing method are known to be major disadvantages. Moreover, the trees from which the wood comes are an important component of natural balance and excessive elimination of these can but affect such balance.
It has already been proposed to simplify the technique used for manufacturing writing pencils by replacing the actual wood with a thermoplastic material selected for its properties of sharpenability which have to be comparable to wood.
In U.S. Pat. No. 2,790,202, said wood material is extruded around a rigid preformed lead, optionally coated with an adhesive in order to improve adherence between the lead and the wood material.
According to a variant embodiment of said U.S. patent, the lead is obtained by extruding a thermoplastic material containing a filler, and the wood material is co-extruded around the lead.
French Patent No. 1 588 294 proposes to produce the lead from a thermoplastic material containing a filler, such as a graphite filled material, and to envelope it with a wood material containing an expanding agent, i.e. a substance whose decomposition occurs within the range of temperatures at which the extrusion of the wood material is performed.
The first role of the expanding agent is to reduce the density of the wood material, the object being to arrive as near as possible to the density of wood itself, which is around 0.5. Moreover, such use of an expanding agent improves the sharpenability of the wood material and reduces its cost.
French Patent No. 2 099 248 describes a method, which is particularly applicable to the continuous manufacture of writing pencils, in which the sheathing with the wood material is performed immediately as the lead material emerges from the outlet of the extrusion die. The technique used here is that of bi-extrusion.
It is easily understood that such a type of method presents very great advantages over the manufacturing methods of the conventional pencils: continuous manufacture, better control of the raw materials, higher productivity.
Nonetheless, the applicant has found that the manufacture of writing pencils which uses bi-extrusion with thermoplastic materials of the type of those recommended in French Patent No. 1 588 294 was particularly restrictive.
Indeed, defects have been noted during the manufacture of certain types of pencils, which defects consist in the presence of bubbles on the outer periphery of the lead material. Such defects are redhibitory as they reduce the strength of the lead and moreover they are detrimental to the appearance of the sharpened pencil.